In the case of soldering electronic components onto a circuit board of an electronic device, there has hitherto been employed a method comprising supplying a solder paste, which is prepared by mixing a solder powder with a flux, on the lands of a circuit board by screen-printing, heating solder paste to form solder layer on the lands, mounting the electronic components on the solder layer prepared above, and melting the solder layer again with heating, thereby jointing the electronic components with the lands through solder.
Due to the size and weight reduction trend in electronics, an electronic component tends to have greater number of terminals with fine pitch. Consequently, the lands of a circuit board also need to array at very small intervals within small area.
It is considerably difficult to supply a solder paste onto the fine pitch lands only by screen-printing. Furthermore, the fine pitch lands might cause solder bridging wherein solder joints two or more conductive parts (i.e. short-circuiting), and thereby it results in poor electrical insulation.
As a method of solder pre-coating on the fine pitch lands, there has been known a method of applying a solder precipitating composition containing a tin powder and a lead salt of an organic acid to wide area including the lands and the space portion thereof, and heating the composition to form tin-lead solder on the lands through the substitution between a portion of tin particle and organo-lead salt under the heat (see U.S. Pat. No. 5,145,532).
However, since environmental pollution due to toxicity of lead has recently become a serious problem, so-called lead-free solder is used for electronics assembly. The conventional lead-free solder alloys in the form of solder pastes are mainly tin-silver, tin-copper and tin-silver-copper solders.
According to the principle of alloy formation by the above solder precipitating composition, it is also possible to obtain the lead-free solder with use of tin powder and a silver salt or a copper salt, where the substitution between a portion of tin particle and such organometallic salts gives lead-free alloys as described above.
However, the lead-free solder precipitating composition has the potential problem in feasible reductive precipitation of organometallic salts to form metal film on the surface of a circuit board (between lands) under the heat. The metal film can not be removed by cleaning so that it makes electrical insulation reliability worse.
Solder alloy formation through substitution between a portion of tin particle and organo-silver salts or organo-copper salts competes with the metal film formation as described above. Therefore, in case that reductive precipitation of organometallic salts would be induced at relatively low temperature wherein the metal film formation proceeds predominantly over the substitution, it is very difficult to adjust final composition of solder alloy.